There are numerous situations where it is important to detect the presence of a human. Examples include within a house, on or near a battlefield, at a border, etc.
There have been prior attempts to accurately and efficiently detect the presence of a human both in enclosures and outdoors. For example, microphones and ground vibration sensors have been used to listen to the sound and vibrations caused by human footsteps. Ground vibration sensors are limited by at least two key phenomena.
The first limitation is the site transfer function. Vibrations produced by forces on the surface of the ground propagate from the source to the receiver in a manner that is controlled by the acoustical soil properties of the ground, the site transfer function. The received signal is the product of the source signal and the site transfer function. For ranges of interest, this transfer function filters the source signal so strongly that it only barely resembles the source signal.
The second important limitation is the vibration coupling of sounds in the air into the ground at the receiver location that either mimic signals that arrive thru the soil or dramatically raise the vibration background level at the sensor.
One example of trying to overcome these disadvantages is the development of sensor packages that combine both microphones and seismic vibration sensors to discriminate between airborne and ground-borne sounds.
Since the physics of the site transfer function precludes a frequency-based detection approach using seismic vibration sensors, various groups have for many years relied on the impulsive nature of human footsteps. See, for example, K. M. Houston and D. P. McGaffigan, “Spectrum Analysis Techniques for Personnel Detection Using Seismic Sensors,” SPIE, Proceedings on Unattended Ground Sensor Technologies and Applications, Vol. 5090, 162-173 (2003). While this impulsive processing does aid in detection, these transducers respond to impulsive sounds in the air that couple into the ground as impulses at the sensor. In this case, the more local site transfer function or the phenomena of acoustic-to-seismic coupling at the transducer resembles the above-mentioned site transfer function and results in similar effects. In effect, impulsive sounds in the air are false alarms to this approach. Even if the transducers were in ideal quiet acoustic environments, humans can walk in a stealthy manner that may limit detection ranges to only a very few meters, at best.
Additionally, microphones are plagued by wind noise, even at low wind speeds. Wind or air currents flowing over the surface of the microphone produce pressure fluctuations that severely limit the microphone's dynamic range or its sensitivity to low-level sounds. While windscreens reduce wind noise, these devices bring their own set of undesirable attributes. Most importantly, windscreens need to be physically quite large to work effectively, are difficult to conceal, and are not designed for long term unattended use in harsh weather environments.
An important microphone wind noise phenomenon is that this noise falls off as the inverse of the frequency. Microphones designed to function in the ultrasonic frequency range will not suffer from this dynamic range limitation, but the source signal that one is trying to detect must be in the high frequency range. Microphones designed with high sensitivity to work in the audio frequency band will always result in signals dominated by wind noise.
Clearly, humans have the potential to detect sounds or the presence of footsteps during windy times. Human ears along with their brain “processor” can detect footsteps and easily distinguish between the footsteps of humans, horses, or dogs, for example. The well-known low-frequency roll-off of the human ear's frequency response or “A-weighting” accounts for the ability to detect during windy times. Human ears reduce the effects of the dynamic range limiting process of wind by not being sensitive to low frequencies. Our ability to detect and discriminate between the footstep sounds produced by humans and animals and even between people results from the impulsive nature and the timing of the sounds produced by the foot's interaction with the ground or the gait of the individual or animal and the signal's' frequency content.